1. Field of the Invention
This invention relates to optical systems including a combined objective lens and fold mirror, in particular but not exclusively for such a lens, for use in an optical scanning device for scanning optical record carriers using a built-in radiation source.
It is desirable to reduce the size of optical scanning devices for use, e.g., in portable devices. It is possible to reduce the size of a device by reducing the build height of the optical system. A typical optical system in an optical scanning device includes an objective lens, which may be a single lens or a compound, i.e., multiple element, lens, and a fold mirror. The build height is determined by the total height of the objective lens and by the height of the fold mirror. In a typical optical scanning device, the height of the fold mirror is greater than the entrance pupil diameter.
2. Description of the Related Art
U.S. Pat. No. 5,432,763 describes a combined single lens objective and fold mirror, in which the build height is reduced. A further such lens is described in German Patent DE-A-19654388.
For relatively high numerical aperture (NA) systems, i.e., NA>0.65, a two-element objective is desired to relax the manufacturing tolerances to acceptable levels.
U.S. Pat. No. 6,058,095 describes two-element high-NA objective lens designs without a combined fold mirror. One of the characteristics of the designs is that the focal distance F1 of the first element facing the radiation source and the total focal distance F of the total system satisfies the relation:
                    1.7        <                              F            1                    F                <        2.5                            (        1        )            
Japanese Patent Application No. JP-A-08201698 describes a combined lens element and fold mirror. The fold mirror is in the form of a curved surface, and the element is to compensate for field curvature. The element is used in combination with an objective lens of NA=0.55.